As known, a laser source has a narrow emission spectrum. The use of the laser source in a projection apparatus is able to result in better color purity and create vivid images with extensive color coverage.
FIG. 1 schematically illustrates the architecture of a conventional laser scanning projection apparatus with phase detection and compensation. As shown in FIG. 1, the conventional laser scanning projection apparatus 1 comprises a laser source 11, a scanning mirror 12, a projection window 13, and a projection surface 14.
The laser source 11 emits a laser beam 15 to the scanning mirror 12. The scanning mirror 12 is a two-dimensional microelectromechanical (MEMS) scanning mirror. When the laser beam 15 is reflected by the scanning mirror 12, the laser beam 15 is transmitted through the projection window 13 and projected on specified locations of the projection surface 14 according to a raster scanning trajectory or a Lissajous scanning trajectory. Moreover, the laser beams with various wavelengths are time-sequentially projected on the target locations in order to create a desired image. Take the raster scanning trajectory for example. The laser beam 15 reflected by the scanning mirror 12 is swept across the projection surface 14, wherein one row of a scan line is projected from left to right at a time and the scanning lines are sequentially projected from top to bottom. Moreover, according to the imaging principle of the human visual persistence, an image frame is displayed on the projection surface 14.
As shown in FIG. 1, the projection window 13 comprises a transmissible part 131 and a blocking part 132, wherein the blocking part 132 is disposed around the transmissible part 131. When the laser beam 15 reflected by the scanning mirror 12 is directed to the transmissible part 131, the laser beam 15 is transmitted through the transmissible part 131 and projected on the projection surface 14, thereby scanning and producing a specified image frame on the projection surface 14. When the laser beam 15 reflected by the scanning mirror 12 is directed to the blocking part 132, the laser beam 15 is blocked by the blocking part 132 and thus fails to be projected on the projection surface 14.
For accurately outputting the image, the projection position of the laser beam 15 and corresponding image information should precisely match each other, so that the laser beam 15 with desired color and intensity can be projected on the correct position of the projection surface 14. If there is a phase difference between an actual projection position and a predetermined projection position of the corresponding image information, the image quality is adversely affected.
For compensating the phase difference between the actual projection position and the predetermined projection position, the conventional laser scanning projection apparatus 1 further comprises an optical sensor 16. The optical sensor 16 is used for detecting the status of the projection position of the laser beam 15 that is reflected by the scanning mirror 12. According to the status of the laser beam 15, the optical sensor 16 issues a sensing signal to a controlling unit (not shown). According to the sensing signal, the controlling unit may calculate the phase difference between the actual projection position and the predetermined projection position. In other words, the sensing signal outputted from the optical sensor 16 denotes the position data or the phase data in the space, and the sensing signal does not only denote the intensity of the laser beam. After the phase difference between the actual projection position and the predetermined projection position is obtained, the subsequent image frame is compensated according to the phase difference.
In the conventional laser scanning projection apparatus 1, the optical sensor 16 is located at a specified position. For example, as shown in FIG. 1, the optical sensor 16 is located at an upper edge of the blocking part 132 of the projection window 13. After the laser beam 15 has been switched on within a specified time period to sweep across the blocking part 132 of the projection window 13, a feedback scan line 17 may be swept across the optical sensor 16. If there is no phase difference between the actual projection position and the predetermined projection position, the laser beam 15 will pass the optical sensor 16 in the specified time period.
If there is a phase difference between the actual projection position and the predetermined projection position, the time period of generating the sensing signal by the optical sensor 16 is shifted. According to the time shift, the phase difference between the actual projection position and the predetermined projection position is calculated in order to compensate the subsequent image frame. Since the optical sensor 16 should be located at a specified position by using this method to calculate the phase difference, the applications of the conventional laser scanning projection apparatus 1 are limited. Moreover, a single optical sensor is only able to detect the phase difference along a single direction (e.g. a horizontal direction). For detecting the phase differences along the horizontal direction and the vertical direction, it is necessary to install two optical sensors on the upper edge and the lateral edge of the blocking part 132 of the projection window 13. Consequently, the fabricating cost is increased.
Therefore, there is a need of providing an improved laser scanning projection apparatus with phase detection and compensation in order to eliminate the above drawbacks.